1. Field of the Invention
This invention relates to improved, disposable paper filters for purification of liquids and gases, particularly water and air, and to methods for manufacturing such filters. The paper filters of this invention are especially suitable for removal of such dissolved impurities as chlorine, organic contaminants and metal cations from drinking water.
2. Description of the Prior Art
In the past, most paper filter media have utilized adsorbents segregated from the paper filter medium itself, either as a separate layer or in a volume enclosed by the paper filter medium. See, for example, U.S. Pat. No. 3,327,859 (Pall), in which the paper filter medium surrounds a sorbent bed of activated carbon particles, which are used to remove chlorine and other contaminants. Other examples include U.S. Pat. No. 4,094,779 (Behrnman); U.S. Pat. No. 4,749,481 (Wheatley); and U.S. Pat. No. 4,039,444 (Bory, et al). Other patents disclose sorbent beds above or below filter sheets. See, for example, U.S. Pat. No. 4,800,018 (Moser); and U.S. Pat. No. 4,054,515 (Sawyer). Commercially available portable drinking water filters incorporating beds of carbon, such as the water pitcher style filter distributed by Brita (USA) Inc., of Oakland, Calif., utilize complex rigid plastic enclosures to contain the coarse carbon particles.
Still other patents, including U.S. Pat. No. 4,747,955 (Kunin) and U.S. Pat. No. 4,505,823 (Klein) disclose filters using admixtures of polymers such as polyesters and polyurethanes with the cellulose base. Some references suggest the incorporation of the maximum possible amounts of various granular materials into paper filters. See, for example, U.S. Pat. No. 4,859,340 (Hou, et al.); British Application No. 2,145,011 (Schroder); British Patent No. 870,975 (Fram) and European Application No. EP 0 402 661 A1 (Dirk).
These prior approaches suffer from a variety of disadvantages. The enclosure of beds of particulates such as clay or carbon inside paper pouches requires fabrication and closure of the pouches, which substantially complicates their manufacture. Only coarse particulates can be used because finer particulates cement themselves together upon contact with water and cause the bed of particulates to lose permeability, making filtration much slower or actually impossible. Moreover, beds of particulates enclosed in permeable filter sheets tend to leak small amounts of fine particles into the water or other fluid being treated, causing turbidity and discoloration.
Portable water filters using plastic filter cartridges, such as the Brita filter, share these disadvantages. In fact, leakage of fines discolors the initial batch of water treated with such devices so much that the initially-treated water must be discarded. Another disadvantage of portable water filters using plastic filter cartridges containing carbon or other adsorbents is that they do not provide any indication of when the carbon becomes saturated with tasteless contaminants such as lead. Therefore, they may continue to be used after they become ineffective. Moreover, such filters typically remain immersed in the water being treated, which may provide a hospitable medium for bacterial growth.
The disposable paper filters of our invention overcome these disadvantages. If particulate additives such as attapulgite or activated carbon are used, they are incorporated directly into the filter itself, with the assistance of novel additions and modifications to the papermaking process. Thus, very fine particulates (which have high surface areas and therefore higher adsorption rates and contaminant carrying capacities) can be used without any cementation of the particles or appreciable discoloration due to leakage into the treated fluid. Because our filters may be disposable, single-use filters, there is no need for any indication of when they become saturated with contaminants and no danger of contaminant breakthrough. And, our filters are suitable for removal of cationic contaminants such as lead, unlike certain prior art filters designed for anionic contaminants.
The paper filters of our invention are fully biodegradable and recyclable. Moreover, embodiments of our invention that do not use any wet strength agent may be flushed down toilets or discarded in plumbed-in garbage disposal units without adverse environmental effects.
Unlike such prior art filters as the Brita filter, the paper filters of our invention are not stored in a wet condition after partial use. They do not need to be activated or reactivated with water, and they do not lose any recovery efficiency if allowed to dry out after partial use.